1. The invention relates to an electronic tuning system for communications apparatus, and in particular for television and radio receivers which employ a phase locked loop (PLL) system for the setting tuner oscillator of the receiver apparatus, which, with the aid of an auxiliary oscillator successively balances during the tuning of the receiver apparatus the individual tuning circuits to optimal values and which coordinate the determined adjustment.
2. Brief Description of the Background of the Invention Including Prior Art
Such a tuning unit is described for example, in German Patent Application Laid Open DE-OS No. 2,854,852, where three auxiliary oscillators are employed and connected to coordinated analog storage units, where phase locked loops are employed to tune the high frequency and radio frequency circuits to the desired receiver frequency. In this tuning unit the tuner oscillator is employed for the balancing. Furthermore, the number of auxiliary oscillators required corresponds to the number of high frequency and radio frequency circuits to be tuned. The number of the required auxiliary oscillators and thus the material requirements are then increased in a particular amount if, for example during the balancing of a television receiver, the band I, the band III and the bands IV and V have to be balanced. Here the danger exists of ambiguities between the frequencies of the tuner oscillator, the auxiliary oscillators and the receiver frequencies. It is disadvantageous for a practical realization of the proposed tuning unit of the reference that the exciter coils of the auxiliary oscillators can cause undesired resonances with the individual coils in the high frequency circuits and that damping of the circuits can be produced. In addition, an uncontrolled detuning can be caused if the auxiliary oscillators are switched off after the balancing has been performed. In addition, analog storage units are provided, which storage units have to be recharged at certain time intervals in order to maintain the optimum tuning values. For these reasons it is in principle not possible to have an optimal balancing of such a receiver apparatus. Also, the continuously repeating adjustments can become visible or, respectively, audible as interferences on the picture screen or the loud speaker.
A tuning method for radio receivers is taught in the journal "Nachrichten Elektronik" Issue 11-79, pp. 365-368, which also employs additional auxiliary circuits in the high frequency filters. There here results disadvantageous influences on the circuits during the operation of the balanced receiver. As set forth on p. 367, left and center column, the accuracy of the tuning is achieved by special components only in the form of exactly paired diodes. This requires equipment expenditures which can not be neglected.
There is also known a test system for a receiver from U.S. Pat. No. 3,736,512, which tests during on-line operation the properties of the individual stages of the receiver such as, for example, the pre-amplifier, the mixing stage, the radio frequency amplifier. In the case of deviations from the set points which are indicated by optical means, these have to be corrected by the operator of the receiver apparatus. The system taught is not a balancing system closed in itself where the radio frequency filtering circuits are balanced automatically to an optimum value for each receiver channel.